Traditionally, a way of recognizing the existence of a possible defect in a tire is by carrying out the so-called “percussion test” which consists in banging with your fist, a hammer or another blunt element the area in question and capture with the naked ear “differences” in the clarity of the sound perceived between two adjacent areas; if this occurs, it is a sign that some disturbance exists. Subsequently, to verify the presence and the extent of the alleged and sometimes inexistent defect, repair workers dig under the area with a reamer and tear the material until verifying through ripping the extent of the damage. Once this is carried out, typically the final digging dimension exceeds the manufacturer's recommended limits for carrying out repairs. Consequently, such a ripped tire is discharged and its remaining rubber is wasted.
In this scenario, every action taken towards introducing in this field new highly technified work practices that efficiently increase the unit performance and reliability is unquestionable and beneficial.
By virtue of the previous explanations, the present invention solves this problem of the art by letting radiofrequency ultrasonic waves or square waves in which interact with the rubber of the OTR giant tires and through their echoes they provide us with defect, degradation state and internal condition data.
Documents WO2004019028A2, JP7103949A and WO1990002946A can be considered similar to the proposed invention with regard to the state of the art.
Document WO2004019028A2 protects a portable apparatus intended for inspection through ultrasound, particularly adapted to examine a container. The apparatus sends an ultrasound pulse to be reflected by the container back wall and obtain data from the echo that was received as an answer and thus determine certain information about the container contents.
The invention mentions a gun-shaped ultrasound apparatus (“Ultrasonic Gun”) that uses two 200 Khz and 1 Mzh frequency transducers designed to identify substances or materials inside a hermetic receptacle or sealed container. Due to the range of frequencies, the apparatus has a very limited, very specific application with reduced versatility. Therefore it can not be used in a tire inspection since rubber is a material with low transparency to ultrasound. Consequently, this instrument technology is insufficient.
This is an ultrasonic inspection apparatus particularly adapted to examine the contents of hermetic receptacles that can not be opened and other dubious origin receptacles. Such contents can be liquid, solid or semi-solid material. The apparatus has the shape of a gun that is held and operated manually. At its end or front end there is one low frequency transducer and one high frequency transducer plus a temperature sensor. This part of the apparatus is the one that is placed in contact with the surface of the container wall to be inspected. A basic technology ultrasound pulse comes out of the gun and enters the container and travels or is transmitted through the wall and liquid or material inside the container. During its passing this initial ultrasonic pulse suffers transformations or modifications either due to the temperature, type of material of the container, time and distance traveled, which in the end will have to do with the container or receptacle dimensions and physical features of the contents. These transformations will be reflected in the back “echo” or modified return signal that arrives at the instrument which in turn, via an electronic processor, delivers the return signal in the form of a specific digitalized wave that should be interpreted by the operator. Obviously, the operator has carried out previous calibrations, entered data for measurements, adjusted parameters, etc.
The great majority of the materials used in engineering are transparent to ultrasound; however, some materials as the “rubber” are more difficult to cross.
Lets remember that ultrasonic waves are acoustic waves identical in nature to sound waves, and the only difference between them is that the ultrasonic wave frequency is high above the audible area: infrasonic, frequencies lower than 16 cycles/sec. (Hz); sonic (audible), frequencies comprised between 16 Hz and 20 (Khz); ultrasonic (non-audible), frequencies higher than 20 (Khz).
Ultrasonic waves make use of elastic properties of a body to propagate, and that is why they require the existence of a material medium (atoms and molecules), i.e., unlike electromagnetic waves they cannot propagate in vacuum.
When an ultrasonic wave reaches a material surface it elastically deforms an atom plane which in turn transmits such deformation to the neighboring atomic planes due to the existent interactions or interatomic cohesion forces. In this way the ultrasonic wave penetrates and travels through a specific body. The energy that is present within an ultrasonic wave creates the oscillatory stress needed to produce the movement of the first plane which is transmitted to the other planes inside the material with a certain speed, typical in each material.
Finally, if a crystal material has practically constant elastic properties, regardless the direction from which a mechanical effect is presented from outside, then we say that this material is “elastically isotropic”, and we talk about a reduced “elastic anisotropy” of the material. The materials with reduced elastic anisotropy are often transparent to sound and as a general rule they can be perfectly verified through ultrasounds. In short we can say that a material with an organized and uniform “texture” has good transparency to ultrasounds.
Nevertheless, as it has been already said, in the case of rubber the situation changes radically. Rubber belongs to the polymer family, particularly to those called elastomers, and in no case these materials account for a perfectly adiabatic, homogenous, uniformly organized medium for an ultrasound transmission. Its molecules are elongated and disorganized and when they are excited, they consume high energy which is mainly attenuated through dispersion because they are deformed in many directions due to their amorphous condition. A three-dimensional structure, which also improves its mechanical properties, is achieved only by a vulcanization process.
Given these difficulties, in the field of Non Destructive Testing, there were only weak and unfinished attempts of rubber testing that were carried out by a few equipment manufacturers.
On the other hand, the present invention, as it has been seen previously, do not lie in the design of an ultrasonic wave generator apparatus or instrument but instead it consists in and seeks to protect the fulfillment of the ultrasonic technique development and the applicability of its parameters in order to detect defects in OTR tires, regardless of a particular ultrasonic equipment or instrument that could be used.
Also, the ultrasonic gun uses 200 Khz and 1 Mhz transducers, thus its functionality is limited to these frequencies. The tire vulcanized rubber, depending on some physical and mechanical features, requires a frequency ranging from 0.3 Mhz to 2.5 Mhz for its inspection.
Only in its high frequency circuit the ultrasonic gun uses the square waveform, i.e., up to 1 Mhz, a condition that absolutely rules out the possibility of using this apparatus in a tire inspection.
The present invention considers the use of a wave without rectification, i.e., in the “radiofrequency” mode, which allows us to display important rectification options and wave mode to select the modality that gives us a more accurate and clearer oscillogram through all scanning thickness. In turn, the ultrasonic gun, among other things, does not have such a significant advantage because it is not an equipment intended for versatility, and it is not designed either for special applications in materials that are difficult to penetrate as the rubber.
“The ultrasonic gun” does not have the screen display option in frequencies up to 25 Mhz., which allows us to carry out the “ultrasonic scanning” comprising significant thickness areas on the tire tread. This approach is used in each inspection to improve testing times.
The accuracy of the ultrasonic testing of a tire depends greatly on the changes in the rubber temperature and the most influencing parameter in this testing is the speed of sound which, as it has already been mentioned, is a specific feature. Furthermore, it has been proven that different speeds of sound are given for the same type of tire which is manufactured by different manufactures.
Theoretically, the vulcanized rubber has a 3.6 km/sec. speed of sound; its density is from 1.1 to 1.6 gr/cm3; its acoustic impedance is from 0.25 to 0.37 gr/cm2-sec. The ultrasonic gun works by entering manually, during its initial adjustment, the speed of sound of the material that is allegedly expected to be found inside but without compensating the variations that could occur as those previously mentioned.
The document WO1990002946A protects an ultrasonic quality control station-type apparatus for conventional tire inspection and defect detection. The device makes the tire turn round at a constant rate and has two transducer devices, one transmission device that directs a plurality of consecutive burstings collimated with ultrasonic energy against the tire face and the reception transducer from one side of the tire.
This is a steady apparatus with hydraulic, electrical and mechanical components fixed on the floor by a certain anchoring system. It has the appropriate size for an ultrasound inspection of conventional tires of vehicles or light duty vehicles that do not exceed certain sizes. It has been intended to be installed during the quality control stage of the production line of a mass-production tire plant; to detect possible typical structural damages of the manufacturing processes. Engineering and development put more emphasis on mechanical equipment design than inspection system technology.
It works as follows: the tire to be inspected is placed on an axle activated by an engine. Fixed to an arm placed on the tire tread, 16 ultrasound emission mini-transducers are installed and on the back side of the tire tread, and interior part of the tire, the same number of receiver transducers are placed and carefully distributed in a fan shape. The tire is turned all they way round at 2 to 3 minute constant rate, synchronized with the collimated ultrasonic energy emission and reception of the transducers. If the presence of a defect is detected in the tire, the mechanism provides an automatic ink marking system of the area in question.
It has a control rack where all resulting data from the inspection is digitally displayed in addition to the appropriate data register.
In turn, the present invention is an ultrasound application aiming to in-situ OTR giant tire inspection (diameter: 3.7 meters; weight: around 5.000 kg or higher) since the invention itself is an ultrasound application technique, ultra portable, with the intervention of an ultrasound skilled analyst and a harness-held instrument weighing around 2.6 kg; it does not require any type of installation. It is apparent that the ultrasonic quality control station of the prior art could not be useful for this purpose because the tires to be analyzed are giant and their emerging defects need to be diagnosed at working site.
The nature of the defects that can occur during the manufacturing process of conventional tires of light duty vehicles are not related to the defect generation or damages produced during a giant tire service.
The causes of defect generation in these large size tires are related to complex stress states which end up with rubber severe cracking with a difficult diagnosis: small cracks or separations are originated in these points where the stress is concentrated that sometimes, after little use, they end up triggering large defects whose detection require “state-of-the-art instruments and work technique having the required electronic advantages to carry out more accurate diagnosis.” Ultrasound analyst-operator workers should be highly skilled with specific and broad theoretical knowledge of material science and defectology.
A small and non-mobile mechanism as the ultrasonic quality control station mentioned above is useless for giant tire inspection since its limited use is adapted and designed for conventional tires. Furthermore, it works with the mechanized GoNoGo system which requires only one person with minimum skills to be operated.
Finally, document JP7103949 A discloses a high sensitive system for tire defect-detection through an ultrasonic test. It uses method wherein ultrasonic pulses are transmitted from an ultrasonic vibrator and only propagate through an ultrasonic medium and a tire.